Space Dementia Wizard Achievement
Solution 1:
I was having difficulty with this achievement (new with Reaper of Souls), trudging all over each act looking for areas with large numbers of weak enemies (pygmies in Act II, scorpions in Act III, etc.) until I stumbled across a surefire way to earn this achievement that I want to share with the community in case anyone else is curious.
Use the Black Hole skill with the Supermassive rune. This extends the area of effect from 15 yards to 20 yards, making it "grab" more enemies in one cast.
Look for a cursed chest. I believe Adventure Mode is best for this, because as soon as you enter an area with a bounty the UI will tell you what the bounty is, and "cleanse the cursed chest" is a possible bounty.
If the cursed chest is the variety "kill 100 enemies before the timer expires" then you are in luck. Kill enemies as normal until you are about 60-70 in. Then mass quantities of enemies will spawn. Stop damaging them and try to let them bunch up on you, then cast Black Hole centered on yourself. If all goes well the achievement will pop. remember, you only need to damage thirty enemies, not kill them.
It may be possible to earn this on Iskatu at the start of Act IV as well but I did not test this.
Solution 2:
The easiest way would be to go to either campaign, beginning of Act IV and face Iskatu, wait for enough enemies to spawn around you and use black hole then.
Another easy way is to go to Act I, The Weeping Hollow and find few of Retching Cadavar mobs that spawn zombies (do not confuse with Deathspitters, they have same model) and run around them until there's enough zombies spawned and then use black hole.